In recent years, along with increase in the capacity of the lithium ion secondary battery, using a material alloyed with lithium as the anode active material is being studied. For example, as such an anode active material, an alloy-based anode material containing an element that can form an alloy with lithium, such as silicon or tin, whose theoretical capacity is higher than that of the carbon material, is being studied.
However, because the volume in the cell is limited, if the alloy-based anode material is used, expansion of the active material cannot be absorbed and pressure is applied to the respective members configuring the battery, so that a problem that damage of the member, deformation, the rise of the cell internal pressure, the lowering of the battery characteristics, and so forth are caused occurs.
Against this problem, a technique of forming a concave and convex shape in the separator to absorb the expansion of the alloy-based anode material has been reported. For example, in Patent Literature 1, a technique of forming a concave-convex shape in a polyolefin microporous membrane by embossing to absorb the expansion of the anode has been proposed.
Furthermore, in order to realize increase in the capacity of the lithium ion secondary battery, increase in the potential of the cathode is being studied. Along with the increase in the potential of the cathode, oxidation resistance and heat resistance are needed for the separator. Against this, a technique of providing a surface layer using heat-resistant particles on the separator and the electrode to enhance the heat resistance and oxidation resistance of the battery has been proposed.